Truly one of the things that causes me to full body shudder is the thought of someone serenading me. If said event actually happens I can't do the shudder because that would be rude, so I sit there with a fake grin and try to avoid an aneurysm. It's difficult, to say the least.
The type of instrument that is being played is of no matter, I just don't want any part of any of that. I'm sure other people like being sang to, I am just not one of those people. I find it incredibly awkward and I don't know what to say or do in that situation.
I saw a movie tonight where this happened. A guy playing a guitar and singing to a girl on a first date (the horror) and I leaned over to my roommate and whispered probably a bit too loudly, "this is my nightmare!". I'm guessing everyone in the theater is now aware of this fun Rachel fact.